1. Field of the Invention
This invention relates to light modulators and more specifically to a charge controlled mirror (CCM) with improved frame time utilization.
2. Description of the Related Art
Beam-addressed light modulators use a scanning electron gun to write a charge pattern onto a pixelized beam-addressing surface of a light valve target. The charge pattern imparts a modulation onto a light beam in proportion to the pixel intensities and directs the modulated light beam through projection optics to form a video display. Such beam-addressed light valve targets have been demonstrated using transmissive and reflective liquid crystals, reflective membranes and micromirror arrays.
Most of these targets utilize the secondary electron emission characteristics of the addressing surface to write the charge pattern. As shown in FIG. 1, the addressing surface is characterized by a secondary electron emission curve 10 that plots the emission coefficients .delta. i.e. the ratio of emitted secondaries to incident primaries, against the landing energy of the primary electrons. At landing energies between first and second crossover points (.delta.=1), 12 and 14, the surface exhibits a coefficient greater than one. Outside that region, the surface exhibits a coefficient less than one. In general, clean conductors have coefficients less than one and insulators have coefficients greater than one for useful beam energies.
In known systems, the write gun emits primary electrons that strike the target's addressing surface with a landing energy above the first crossover causing more secondary electrons to be ejected than incident primary electrons. The secondaries are collected by a collector electrode (grid or plate), which is held at a relatively positive potential with respect to the addressing surface. This produces a charge pattern that has a positive net charge, which increases the pixel potentials and in turn actuates the liquid crystal, membrane or micromirror to modulate the light. The degree of modulation is controlled by changing the beam current.
The write gun preferably operates above the first crossover and below the second crossover to reduce the amount of beam current that is required to write a given charge pattern. Surface coatings such as MgO or doped diamond like films that exhibit stable emission coefficients in the range of 5-50 for useful beam energies are commonly available and act as a current amplifier. Some surface materials such as clean aluminum have .delta.&lt;1 (0.7-0.9), and provide no current application for the primary beam. Current amplification is quite important given the nanosecond dwell times available at normal video rates for each pixel.
In video applications, each charge pattern or frame must be erased prior to the next pass of the write gun. It is well known that the brightness of the light modulator is closely tied to the optical throughput of the target. In large part, optical throughput is determined by the frame time utilization of each pixel, i.e. how long the pixel is held in its modulated position before it is erased. Ideally, each pixel would be held at its intended modulated position until that pixel was to be rewritten and then instantaneously erased. This would maximize the amount of light passed through the projection optics while maintaining video performance.
A common erasure technique is RC decay, in which the deposited charge is bled off over the frame time. The device's RC time constant must be short enough that the pixel intensity is erased prior to writing the next value in order to maintain video performance. The main drawback, however, is the fact that approximately two-thirds of the available light is lost due to RC decay. This greatly limits the display's brightness and contrast capabilities.
In the early 1970s, Westinghouse Electric Corporation developed an electron gun addressed cantilever beam deformable mirror device, which is described in R. Thomas et al., "The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays," ED-22 IEEE Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos. 3,746,310, 3,886,310 and 3,896,338. A low energy scanning electron beam deposits a charge pattern directly onto cloverleaf shaped mirrors causing them to be deformed toward a reference grid electrode on the substrate by electrostatic actuation. Erasure is achieved by raising the target voltage to equal the field mesh potential while flooding the tube with low energy electrons to simultaneously erase all of the mirrors, i.e. the whole frame. This approach improves the modulator's FTU but produces "flicker", which is unacceptable in video applications.
More recently Optron Systems, Inc., as described in Warde et al., U.S. Pat. No. 5,287,215, has developed a membrane light modulation system in which a charge transfer plate (CTP) couples charge from a scanning electron gun under vacuum through to potential wells in atmosphere. A deformable reflecting membrane is supported on insulating posts and spans the wells. The CTP serves as a high-density multi-feedthrough vacuum-to-air interface that both decouples the electron beam interaction from the membrane and provides the structural support required to hold off atmospheric pressure.
Warde suggests two ways to write and erase the CTP. The first is very similar to the Westinghouse technique in that the membrane is switched to the grid voltage and rescanned to erase the charge pattern, which Warde acknowledges produces image flicker. The second flickerless mode of operation, which Warde refers to as grid-stabilized, applies the video signal to the membrane and fixes the beam current. This can work for low resolution displays but becomes very difficult at high resolutions due to the capacitance of the membrane. Also, voltage swing requirements are higher, approximately 50V versus less than 10V, which combined with the increased capacitance drive the video amplifier power requirements up 25 to 2500 times.
An electron beam addressed liquid crystal light valve of the cathode-ray tube type is described in Duane A. Haven, IEEE Transactions on Electron Devices, Vol. ED-30, No. 5, 489-492, May 1983. The light valve of Haven is a form of cathode-ray tube (CRT) having a twisted nematic liquid crystal cell, one substrate surface of which serving as a target for a writing electron beam propagating in the tube. The target substrate comprises a thin sheet of dielectric material and forms one face of the liquid crystal cell.
The CRT also includes a writing electron gun, a flood electron gun, and a ring-type collector electrode positioned adjacent the periphery of the target surface. The flood electron gun maintains the target surface of the cell at a desired operating electrostatic potential VFG, which is the potential of the flood electron gun cathode. Polarized light propagating from an external source enters the CRT through an optically transparent entry window on one side of the tube and passes through the cell and out through an exit window. The writing and flood guns are mounted at oblique angles relative to the target substrate to keep them out of the light path. Unwritten areas of the liquid crystal cell remain in an "OFF" state that rotates by 90 degrees the polarized direction of the light emanating from the external source. Areas addressed by the writing beam are temporarily switched into an "ON" state that leaves unchanged the polarization direction of the light emanating from the external source. This creates a light image pattern that is detected by an analyzing polarizer positioned in the path of light exiting the exit window.
The transparent collector electrode of the light valve of Haven is operated at a potential VCOL, which is positive relative to the potential VFG of the target surface. The flood gun electrons strike the target surface with an energy that is below the first crossover point on the secondary electron emission ratio curve for the dielectric material forming the target surface. Under these conditions, the electrostatic potential of the target surface is stabilized to the potential of the flood gun cathode. The writing gun is operated under conditions so that the writing beam electrons strike the target surface with an energy that is above the first crossover point but below the second crossover point of the dielectric material.
When the writing beam strikes the target surface, secondary emission causes the written area to charge positive relative to the unwritten areas of the target surface, which are at the flood gun potential VFG. The potential of the written area rises, approaching the potential VCOL of the collector electrode and driving the liquid crystal cell into the "ON" state. After the writing beam is turned off, the potential drops back to the flood gun cathode potential VFG and allows the liquid crystal cell to relax to the "OFF" state. This occurs because VCOL is below the first crossover point and more electrons are absorbed than are emitted from the previously written area.
The ring-type collector electrode is positioned adjacent the periphery of the liquid crystal cell and outside the projection light path through the valve. There is a relatively large separation between the collector electrode and the central areas of the target surface, Which separation causes the collection of secondary electrons emitted from the central areas on the target surface to be relatively inefficient. The reason for such inefficiency is that secondary electrons emitted from the central areas on the target surface redeposit on the positively charged, previously written areas of the target surface. This redeposition of secondary electrons at least partly erases the written image, thereby reducing the resolution and contrast capability of the light valve.
A different technique for erasing a beam addressed liquid crystal light valve is presented in U.S. Pat. Nos. 5,765,717 and 5,884,874. The transparent collector electrode is segmented into four or more electrically isolated segments. As the erase and write guns raster scan the light valve, with the write gun lagging by two segments, a controller switches the potentials on the segments above the erase and write guns to ground and to +300V with respect to the incident surface. Since both guns operate at energies above the crossover point, the erase gun secondaries will redeposit themselves over the segment thereby erasing the charge pattern and the write gun secondaries will be collected by the segment thereby writing a new charge pattern.
Although this approach provides improved resolution and contrast, it requires a segmented grid and a synchronized controller. Since both guns operate above the crossover, image resolution can be further improved by coating the entire surface of the LCLV with a material such as magnesium oxide (MgO), which exhibits a very high emission coefficient, as described in U.S. Pat. No. 4,744,636. However, the best FTU that can be achieved using the segmented grid is (n-2)/n where n is the number of segments. For example, a 4 segment grid would have only a 50% FTU.